Rodney LeVake has again failed in his effort to argue that he had free exercise, free speech, and due process rights to teach "evidence against evolution". On May 8, 2001, the Minnesota Appeals Court supported the summary judgment dismissal decision of the Minnesota District Court of last year (see RNCSE 2000; 20 [1-2]: 13-14).

It is unclear on what basis LeVake argues that his right to free exercise of religion was violated. LeVake does not contend that respondents prohibited him from practicing the religion of his choice. He does not assert that respondents demanded that he refrain from practicing his religion outside of the scope of his duties as a public school teacher in order to retain his teaching position, and he does not assert that the curriculum requirements incidentally infringed on his religious practice.

Regarding the free speech argument, the Court supported the right of the district to determine curriculum, a position supported with abundant case law:

The classroom is a "marketplace of ideas," and academic freedom should be safeguarded. But LeVake, in his role as a public school teacher rather than as a private citizen, wanted to discuss the criticisms of evolution. LeVake's position paper established that he does not believe the theory of evolution is credible. Further, LeVake's proposed method of teaching evolution is in direct conflict with respondents' curriculum requirements. Accordingly, the established curriculum and LeVake's responsibility as a public school teacher to teach evolution in the manner prescribed by the curriculum overrides his First Amendment rights as a public citizen. [Citations omitted.]

Regarding the due process claim, the Court wrote:

The school board may regulate a teacher's speech in the classroom if it has provided the teacher with specific notice of what conduct is prohibited. LeVake's due process claim is premised on his belief that respondents deprived him of his liberty interest to teach his class free "from state action which impinges upon and violated his constitutional rights to free speech and free exercise" by failing to provide him with adequate notice of what types of expression were prohibited before reassigning him. The cases LeVake relies on in making this argument involve the termination of teachers, but LeVake was not terminated. In fact, he was not even demoted. Further, before accepting the position to teach tenth-grade biology, LeVake understood that respondents' prescribed curriculum included teaching students about evolution. LeVake was given sufficient notice about what he could and could not teach through the established curriculum and the syllabus. [Citations omitted.]

Concluding the decision, the Court wrote:

Because LeVake's position paper and his statement to Hubert make it clear that LeVake would not teach the required course curriculum in the manner established by the school board, LeVake has not presented any genuine issue of material fact regarding his free exercise, free speech, and due process claims. Thus, the district court did not err in granting respondents' motion for summary judgment.

In 1999 I attended a meeting near Chengjiang, China on "The Origins of Animal Body Plans and Their Fossil Records" (Chen and others 1999). The meeting was held near Chengjiang because there are fantastic fossils of Early Cambrian animals in the area. What makes these fossils remarkable is that they not only preserve only the hard, skeletonized parts of animals, but also contain replicas of the external form of their soft parts. Such sites of such "exceptional preservation" are rare, but are enormously important scientifically for the glimpses they give us of the panorama of early animal life (Gould 1989; Conway Morris 1998).

Although the meeting was billed as a scientific conference, a number of anti-evolutionists were also in attendance, including several people associated with the Center for the Renewal of Science and Culture (CRSC), the creationist arm of the Discovery Institute, a Seattle-based organization that advocates "intelligent design" as an explanation for biotic diversity. Indeed, one of the principal organizers of the meeting was Paul Chien, a marine toxicologist at the University of San Francisco in Santa Rosa, California, and a senior fellow of the CRSC. The talks were scheduled to provide prominent slots for CRSC fellows and their associates. Even more troubling was the fact that scientists were not informed of the involvement of the CRSC before arriving at Chengjiang and only became aware of its involvement once they inspected the printed abstracts of the presentations. Many scientists are becoming concerned about the activities of the CRSC, and so it seems important to clarify what occurred at this conference.

My experience at the meeting convinces me that all the anti-evolutionists who attended were motivated by political, not scientific, interests. There is nothing inherently wrong with that, of course, although it is markedly unusual in a scientific conference. I even had to admire the nerve of several anti-evolutionist speakers who made presentations in front of opponents as formidable as Eric Davidson, the developmental biologist from the California Institute of Technology and a member of the US National Academy of Sciences. Nevertheless, I could find nothing in any of their presentations that provided scientific evidence suggestive of the action of an intelligent designer, undiscovered natural laws that govern the development of form, or the action of some unspecified principle of "harmony" that drove the early evolution of animals.

What was presented were old Paleyian arguments for design cast up in a variety of molecular guises. These arguments are based on the notion that, because we do not currently understand all aspects of the evolution of life, evolutionary ideas must therefore be fundamentally flawed and, therefore, there must be an intelligent designer. Rather than presenting a coherent argument for the action of an intelligent designer, these advocates were more interested in exploring what they present as weaknesses in evolutionary thinking. Their position ignores the colossal amount of concordant evidence supporting evolution and refuses to acknowledge the legitimate complexity of modern evolutionary thinking.

Unlike the contingent of scientists, the anti-evolutionists at the meeting all appeared to have known of the CRSC's involvement with the meeting before their arrival. These anti-evolutionists represented a broad range of anti-evolutionary viewpoints. At one extreme their camp included Michael Denton, a senior fellow of the CRSC, whose professed "pagan" personal philosophy, as he explained it to me, seemed about as far from biblical literalism as one could imagine. At the other extreme there was a young, fervently Christian student who spoke candidly of his belief in young-earth creationism. Denton is a qualified geneticist, and I could trace no scientific or philosophical link between his platonic notions of natural laws that govern the form of animal archetypes and the strict biblical-literalist, young-earth stance espoused by the student. The fact that both these individuals claimed that the Discovery Institute had financially supported their attendance at the meeting suggests that the Institute's involvement in the meeting was not motivated primarily by the desire to present a coherent scientific argument for "intelligent design". Rather, it suggests that the Discovery Institute was more interested in supporting any views that appear to challenge evolutionary explanations, regardless of whether these views are mutually exclusive.

Although I cannot read the minds of the individuals associated with the CRSC, I can advance a reasoned interpretation of what might be motivating their interest in this issue. The US Supreme Court ruled in Edwards v Aguillard (1987) that creationism cannot be taught in science classes in public schools because it is not a scientific concept and because there is no secular purpose in teaching it. Advocates of intelligent design creationism (IDC) may attempt to circumvent this ruling by arguing that "intelligent design" is a scientific alternative to evolution. Tactics for doing so might include having IDC representatives speak at scientific meetings alongside recognized scientists and having their ideas published alongside scientific papers in conference proceedings. The association of "intelligent design" with legitimate scientific conferences and publications could be presented to US lawmakers as evidence that advocates of IDC are pursuing reputable science rather than sectarian politics, and that the scientific community accepts "intelligent design" as a viable and productive topic of study. This in turn would support the recent debate about "alternatives to evolution" to public school science classes. The remainder of this article seeks to identify specific positions that those associated with the Discovery Institute appear to take with regard to scientific evidence on this matter, and then respond to these positions from my own viewpoint as a scientist.

Why Anti-evolutionists Focus on the Origin of Animals and The "Cambrian Explosion"

It seems likely that the origin of animals will remain a favorite subject for anti-evolutionists over the coming years. Before reviewing the current scientific evidence concerning the origin of animals, I wish to outline the strategic position of IDC anti-evolutionists — especially those associated with the CRSC — with regard to the origin of animals as far as I understood it from the meeting.

IDC advocates claim that: 1) The major groups of animals had separate, independent origins (by "major groups of animals" anti-evolutionists mean the marine creatures without backbones that commonly correspond to the "invertebrate phyla" such as Mollusca, Brachiopoda, Arthropoda, and so on ... in addition to the first Chordates). This position, of course, denies common ancestry among living taxa. 2) These major animal groups originated over a "very short" interval of geological time associated with the "Cambrian explosion" (some IDCs suggest a period of 2-3 million years). This is much shorter, of course, than expected by most evolutionary models.

These two points lead to the conclusion that the rate and magnitude of innovation were far too high to be accounted for by natural selection and can only be explained as the actions of a designer.

The IDC position is challenged by the scientific facts. The claim that the major groups of animals appear suddenly in the fossil record is easily demonstrated as incorrect by the extensive fossil record of early animal evolution that stretches back several tens of millions of years earlier than the Chengjiang fossil beds. So anti-evolutionists are in deep denial of the fossil record when they cite the impossibly short interval during which new taxa emerged.

The claim that the major animal groups originated separately and independently is equally weak. The origin of animals took place relatively late in the earth's 4.6-billion-year history; however, recent fossil discoveries contain evidence of a long and rich ancestry for animal phyla. Of course, when most people think of the "major groups" of animals, they envision the vertebrate classes that represent what first think of as "animals" today — mammals, reptiles, amphibians, fishes, and birds. Vertebrates represent only some of members of one phylum — the Chordata. There are numerous other phyla, represented by such disparate creatures as starfish (Phylum Echinodermata), spiders and crabs (Phylum Arthropoda), clams and snails (Phylum Mollusca), in addition to many others.

In a broad sense each different phylum has a distinctly different fundamental structure and development — and each represents a different "body plan", or set of "body plans". Scientists understand body plans to refer to the major features of adult bodies in metazoans and/or of the developmental trajectory that gives rise to the adult body. Different body plans are distinguished on the basis of variation in features such as aspects of skeletal construction, symmetry, internal body cavities, segmentation patterns, and appendage structure.

Almost all metazoan phyla can be instantly distinguished from one another on the basis of variation in these fundamental features. The animal taxa that emerged out of the Precambrian are clearly related to living taxa, but even the early chordates — the founding members of the phylum that would later give rise to the vertebrates — bore little superficial resemblance to the vertebrates that would finally appear 100 million years later.

The IDC conclusion that the Cambrian Explosion can be explained only by reference to an intelligent designer is unsupported by the scientific evidence, as discussed below. Moreover, the common anti-evolutionist strategy of quoting Darwin as if science has stood still since he wrote On the Origin of Species backfires. One of the most remarkable aspects of the Origin is the way in which Darwin identified potential criticisms of his theory and addressed them with refreshing honesty. He was very frank about what he thought was the absence of fossils in rocks older than those bearing the oldest skeletonized fossils known in Europe (now known to be about 525 million years old), recognizing this absence as a "valid argument against the views here entertained" (Darwin 1859 [1964: 308]). After admitting that he had no solid explanation of the absence of these fossils, Darwin advanced some hypotheses about the incompleteness of the geological record. It is instructive to compare what we now know about the early history of life on the planet with what was known in Darwin's time and to ask how his views have stood the test of time.

The scientific issues relating to the origin of animals and the Cambrian Explosion

What defines an animal, and how are different animal groups related?

The time around the Precambrian-Cambrian transition is important because it provides us with the first fossilized record of metazoans — multicellular animals with features such as differentiated organs and tissues — about 544 million years ago. I emphasize that "appearance" is not the same thing as "origin". There are myriad reasons related to the preservation and recovery of fossils that can explain why the first recorded appearance of a particular group can occur substantially after its evolutionary origin. The origin of animals is a complex issue for which several independent lines of evidence need to be investigated.

First, we need to decide what it actually is to be an animal. To do so, we must identify unique characters that are shared by all animals and distinguish them from other types of organisms. These are the characters that diagnose animals as a natural group and are considered ancestral for all animals. Once we have done that, we can proceed to identify other novel characters that distinguish specific subgroups (or "clades") of animals from the ancestral state and from each other. This process of distinguishing groups within groups produces a hierarchical nested set of related animals.

Animals are multicellular organisms that have cells specialized to perform particular functions; these cells are held together by an organic glue called extracellular matrix (ECM). On the basis of these features, biologists have long inferred that all animals constitute a natural group and evolved from a single common ancestor. But multicellularity with specialized cells is a general similarity — one that applies to some organisms, such as trees, that we would all agree are not animals — so multicellularity with specialized cells is not enough to prove common ancestry of all animal taxa. These features may have evolved independently in different lineages of single-celled organisms, and so it is the highly specific shared features, such as the nature of ECM, that assume a special significance for defining what it means to be an animal. This is because very specific similarities are unlikely to have arisen convergently and so point towards a single common ancestor for all animals. Recent discoveries of numerous very specific properties shared by all animals provide extremely strong evidence for their common ancestry.

We now know that all animals share not only general similarities but also many highly specific genes, for example, the transcription factors of the ets gene family, paired-box genes, and a primordial Hox gene (Peterson and Davidson 2000). These genes are fundamental in organizing the layout of animal bodies, and have such similar molecular structures that we can confidently conclude that they result from common ancestry, rather than from later evolutionary convergence. Thus they provide extremely strong evidence that all animal groups arose from only one lineage of single-celled ancestors.

The simplest animals, sponges, have all the characters mentioned above (along with a few unique characters of their own), but they lack the next set of features that diagnose an evolutionarily clade of animals derived somewhat later. In accordance with the nested hierarchy of characters we expect, all animals, except the sponges, to exhibit embryonic gastrulation (a special infolding of the wall of the initial ball of cells formed after fertilization), and the duplication of the primordial Hox gene. These features form the basis for diagnosing a more derived group of animals that includes corals and all other metazoans (ourselves included), but excludes the more basal clade that contains the sponges. At each step of the evolution of animals, we can demonstrate a similar diagnostic branching.

What we have just done is to use the distribution of novel features to map the evolutionary changes that both diagnose what makes an animal in the first place and tell us how animals within the group are related. The resultant hierarchical nested sets of related animals are exactly what we would expect according to an evolutionary model. By any reasonable evaluation, these must be considered strong evidence of the evolutionary relatedness of all animals. Of course, the spate of new information on molecular structures and developmental genetics raises many new questions, but the big picture is that these new data can only be viewed as furnishing wonderful vindication of Darwin's central notions. Time and time again, we find that animal subgroups thought to be related on the basis of morphological evidence also share exclusive similarities in gene sequences and in patterns of developmental control, as predicted by evolutionary theory. At the Chengjiang meeting, the CRSC's Jonathan Wells suggested that developmental genetic evidence favors separate origins from different single-celled lineages for the major animal groups. But his suggestion contradicts a wealth of scientific evidence and therefore must, in my view, arise from non-scientific convictions.

Our understanding of the major relationships among animal groups is now stabilizing. This is not to say that we currently know all there is to know about animals' relationships or molecular biology — far from it, which is why evolutionary biology is an exciting area of research. Anyone with knowledge of developmental biology and sufficient time can find aspects of specific systems or pathways the operation and evolution of which are not now fully understood. But what we do understand strengthens the case for evolution because new insights and methods of analysis, unthinkable in Darwin's day, yet again fulfill the predictions of the evolutionary model.

What are the implications of the pattern of relatedness discussed above for evolution and for intelligent design creationism? First and most important, it clearly falsifies the IDC claim that animal groups appeared separately and independently. Instead, we see a hierarchy of the distribution of shared features — some general to the group as a whole, others specific to particular subgroups. Such a distribution of features is predicted by evolutionary theory, which was proposed long before most of these features had been recognized. It is not concordant with the idea of multiple independent origins of animals, because that model would have no compelling basis on which to predict a hierarchical arrangement of such shared features.

An apologist for "intelligent design" could argue that a designer worked sequentially in a series of small steps, which could explain why the features defining clades are arranged hierarchically. Curiously, however, members of the CRSC apparently do not apply this explanation to the Cambrian biota. Rather, they persist in asserting the independent origins of different animal groups, despite overwhelming scientific evidence against that viewpoint.

The extensive Precambrian fossil record

The second pillar of the IDC position — that major groups of animals appeared too quickly for natural processes to account for them without invoking the intervention of an intelligent designer — is equally unsupported by the scientific evidence. However, to expose the weakness requires some background information about the Precambrian-Cambrian transition.

The earliest fossils currently known occur in rocks from western Australia that date from around 3465 million years ago (Shopf 1993), and a reasonably good fossil record is known from that time onwards. The earliest chemical evidence of life itself is even older, about 3900 million years ago (Mojzsis and Harrison 2000). The earliest fossils are prokaryotic cyanobacteria, and as we move up through the geological column toward the base of the Cambrian these forms are joined by more complex fossils, such as those of eukaryotic cells, by about 1800 million years ago (Knoll 1992). (It is important to reiterate that the first occurrence of a fossil marks the minimum age for the appearance of the group to which it belongs, but the origin of the group often is far earlier, as Darwin suggested. For example, there is good chemical evidence that eukaryotes existed from about 2700 million years ago [Brocks and others 1999], but the earliest fossils yet found that are widely accepted as eukaryotes are some 900 million years younger.) The increase in complexity and diversity of fossils through the Precambrian up toward the boundary with the Cambrian is concordant with an evolutionary explanation, and the sequence of appearance makes sense in evolutionary terms. Darwin would be justified in feeling vindicated by these discoveries of definitive Precambrian fossils, which were unknown at the time he was writing. Anyone who suggests that Darwin's 19th-century difficulty with an apparently abrupt start to the fossil record still pertains today simply has not considered the evidence.

When and how quickly did animals first appear, and did all "major groups" appear at the same time?

We do not yet know exactly when the first animals originated because we do not yet know exactly when the definitive characters of animals — extracellular matrix, the primordial Hox gene, the ets gene family, and so on — originated. There is a wide range of estimated dates for the differentiation of the major groups of animals from one another. Some studies suggest that this occurred as much as 1100 million years ago; others suggest a date closer to 600 million years ago (see Valentine and others 1999).

Although the methods used in these estimates are not currently as precise as they may yet become, it is hardly a surprise that the dates for the divergence of major groups are spread over a wide timespan. This is because major groups of animals are related in a hierarchical fashion, as we saw above, and would thus be expected to diverge from an ancestral lineage at different times. Therefore, we would expect the split between vertebrates and echinoderms, groups that share a wide range of derived features, to have occurred more recently than, say, the split between sponges and the common ancestors of echinoderms and vertebrates. Why? Because sponges are among the most basic animals with the fewest derived features, and so we would expect them to have split off earlier. And this is exactly what we do find — sponges and other animals are estimated to have separated about 950 million years ago, whereas echinoderms and vertebrates split from each other somewhere between 700 to 550 million years ago (Smith 1999).

Three lines of evidence provide important constraints on estimates of when the key events in animal evolution happened. Although the three lines of evidence are independent of one another, the results of each approach are concordant with the inescapable conclusions that the origin of animal groups was a protracted affair that required at least 100 million years and possibly far longer, and that the origin of animal groups took place in the Precambrian, long before the "Cambrian Explosion". Thus the IDC position that the origin of animals occurred very quickly as part of the Cambrian Explosion is falsified by these lines of evidence.

The three lines of evidence are:

1. Molecular clocks. If we can estimate the rate at which particular organic molecules change among living groups whose divergence times are well known, then we can compare the amount of difference in the same molecules among a wide variety of animals to calculate approximately when these forms diverged. The estimated rate of change in these molecules is the basis for molecular clocks, and the relationships suggested by multiple molecular clocks demonstrate concordant patterns. We now know that the major novel characters that distinguish the major groups of animals appeared at most 1500 million years ago, but at the latest no more than about 560 million years ago — 15 million years before the start of the Cambrian Period (Lynch 1999; Smith 1999), and about 40 million years before the age of the fossils of Chengjiang.

2. Evidence from body fossils. There is a substantial Precambrian fossil record of animal bodies and body parts. For example the calcareous tube Cloudina represents the outer skeleton of an animal, and has long been known from Precambrian rocks at least 550 million years old. There is undisputed evidence of fossil sponges dated about 545 million years ago (Brasier and others 1997), right about at the Cambrian boundary (544 million years ago), but there are also fossils of sponge embryos dated at around 580 million years ago (Chen and others 2000). Some scientists have also argued that some members of the Ediacaran fauna — an enigmatic suite of late Precambrian body fossils about 555 million years old — represent a variety of animal groups with representatives living today. There is widespread agreement that at least some of these forms represent sponges or cnidarians (jellyfishes, corals, sea anemones, and hydras), but some scientists argue that arthropods and mollusks are also present in the Ediacaran assemblages. Moreover, Chen and others (2000) recently claimed to have recovered embryos similar to those of derived groups such as arthropods and echinoderms in deposits about 580 million years old. These researchers argue that early animal evolution took place at small, almost microscopic sizes, unlikely to leave much of a fossil record. If these interpretations of the Precambrian fossil record are correct, they strengthen the case for argument that the differentiation of the "major groups" occurred much earlier than their dramatic appearance in the Cambrian Period.

3. Evidence from trace fossils. Trace fossils are evidence of the activity of animals, such as the burrows, tracks and trails that animals left on the sediment surface or beneath it in the seafloor. There is no serious argument that large trace fossils were formed by anything other than animals, although there is some debate as to which are the earliest trace fossils, because many simple trace fossils, such as might be formed by the earliest animals, are easily confused with other structures produced by inorganic processes. One thing is very clear: there are many trace fossils in Precambrian rocks at least 555 million years ago, and possibly far earlier (Budd and Jensen 2000). It is also clear that the order of appearance of trace fossils proceeds sequentially from simple to more complex forms (Budd and Jensen 2000). Indeed, one feature that identifies the Cambrian period in the geological is the appearance of the burrow network Treptichnus pedum. The distinctive form of this trace leaves no doubt that it was formed by an animal with a central gut and a reasonably sophisticated neural system. T pedum first appears some 10 million years before skeletonized fossils become common and about 20 million years before the Chengjiang fauna lived (about 525 million years ago).

What does all this mean? The lineages that include the major groups of animals (each major group being characterized by a particular "body plan") certainly diverged during the late Precambrian and not during the Cambrian itself. Because major animal groups share so many developmental features, these features must have originated before these lineages split — that is, at least 580 million years ago, some 45 million years before the beginning of the Cambrian Period. Fossil evidence, both from trace and body fossils, is consistent with this interpretation, and the trace-fossil record suggests the stepwise acquisition of increasingly complex behaviors from about 555 million years ago onward. Hence, any suggestion that the appearance of the first representatives of "modern" animal groups in the Cambrian correlates specifically with the time of origin of these groups (an argument favored by proponents of IDC) is clearly refuted by the evidence, which shows that the major groups of animals originated during the Precambrian.

It is also important to appreciate that about 20 million years pass from the beginning of the Cambrian to the time of the Chengjiang fauna — the Burgess Shale fauna (discussed in Gould's Wonderful Life [1989]) is even more recent. Because Treptichnus pedum, the marker for the start of the Cambrian, was made by an animal with a gut and complex behavior, it is certain that large animals with 3 layers of cells (the triploblasts) were living long before the debut of the stars of Wonderful Life. Any suggestion that animals evolved within "a mere 2 or 3 million years" (Heeren 2000) of the Chengjiang fauna is an irresponsible and bizarre misrepresentation that flatly contradicts scientific facts.

What, then, does the Cambrian Explosion represent?

The Chengjiang fauna, like that of the Burgess Shale and several other Cambrian sites, is truly remarkable for the quality and the range of biological diversity that it preserves. Detailed work on these faunas has revealed a remarkable fact — that animals related to the major living groups of animals were present from at least the later portion of Early Cambrian Epoch. In Wonderful Life Gould (1989) made much of this important fact by suggesting that because forms comparable to major groups of living animals were already present in the Cambrian, later evolutionary history has mostly involved variation on established themes, rather than the origin of really major new animal body plans. He also stressed that several fundamentally distinct animal body plans present in the Cambrian have since vanished. These body plans are found both in lineages that belong to existing phyla (for example, some extinct groups of Arthropoda) and in lineages that seem allied to other metazoans but are obviously not members of living groups (for example, the Archaeocyatha). Thus, according to Gould, most of the fundamental innovations in body plan were in place by Middle Cambrian time, and the Cambrian fauna was more diverse than its modern counterpart.

New discoveries and interpretations in paleontology and developmental genetics have changed the scientific landscape significantly since Gould wrote Wonderful Life. Wills and others (1994) have suggested that Gould may have overestimated the diversity of Cambrian animals, although scientists disagree on how best to measure this diversity. Nevertheless, Gould's central point — that at least some groups of Cambrian animals exhibit a morphological diversity that is at least comparable to that seen in living fauna — remains valid and should not be underestimated. Compared with the fauna of the later Precambrian, the Cambrian fauna is strikingly diverse; the recent discoveries of early vertebrate-like fossils in the Chengjiang beds simply emphasize the point that much innovation was in place by relatively early in the Cambrian Period.

The aspect of Gould's views that has been most strongly challenged is the idea that several fundamentally distinct animal body plans have vanished since the Middle Cambrian. The definition of what constitutes a "fundamentally distinct animal body plan" is difficult because it requires an evaluation of the evolutionary "weight" or significance of particular features; we are not yet sure how to assess this weight objectively. But what has become clear recently is that the Burgess Shale and Chengjiang faunas contain not only members of "crown" groups (those with living representatives), but also animals in "stem" groups, which are more distant relatives of these surviving groups (Budd and Jensen 2000).

Because species belonging to stem groups are typically extinct, their place in the phylogenetic tree can be difficult to interpret. It turns out that many of the forms Gould interpreted as representing additional fundamentally distinct body plans may merely be evolutionary adventures or "experiments" within the lineages of the major groups of animals we know today. So, the unusual fauna of Chengjiang and in the Burgess Shale likely represent way stations along the road to the establishment of the modern groups rather than cul-de-sacs of evolutionary innovation. Some forms we can easily recognize as linked to living groups; others are more enigmatic (and stem groups, of course, show evidence of transitional states). But such difficulty in interpreting early fossils, of course, is what Darwin predicted in the Origin of Species, because he knew that the selective action of extinction throughout geological time could only tend to emphasize differences, not similarities, among these major lineages of animals.

The bottom line is that the establishment of modern animal groups was a protracted affair that began no later than about 600 million years ago, extended across the Precambrian-Cambrian boundary, was still in progress during the Early Cambrian Epoch, and continued after the close of the Cambrian Period. Accordingly, science currently tells us that there was, at a minimum, about 100 million years from the time when the first sponge-like animals originated until the origin of representatives of all the major living lineages or body plans.

Because animals did not evolve in a geological instant, there is no need to invoke some novel evolutionary — or supernatural — process to explain their appearance. This is not to say that the appearance of every novel feature is of the same importance for later evolution or that the rate of appearance of novel features was constant throughout the entire interval. Scientists do not demand such restrictions, even though anti-evolutionists frequently present them as basic premises of evolutionary explanations.

It is clear that there was a fundamental transition that took place over an extended interval across the Precambrian-Cambrian boundary. That transition reflects a dramatic shift in the structure of the ecosystems of early animals, which must have been at least partly fueled by the appearance of new biological innovations. Although a rich mixture of modern groups and their early relatives may have persisted throughout the Cambrian, there is a clear contrast between these generally familiar forms and the more enigmatic fossils from the Precambrian, such as those from Ediacara in Australia. It is clear that the transition into the Cambrian marks a pivotal time in life history. But although much tinkering went on in the Cambrian and thereafter, the most fundamental steps in the origins of animal groups took place during the Precambrian.

To return to Darwin, how does our current knowledge affect his "difficulty" with the "Sudden appearance of groups .... in the lowest known fossiliferous strata"? The answer is clear: Darwin's difficulty has evaporated. We have now identified thousands of fossils that appear earlier in the fossil record than the point at which Darwin thought it suddenly began. Darwin suspected that the impression of sudden appearance was false, and speculated that the false impression was due to the poor preservation of the older rocks in Europe and inadequate attention given to the fossil record. He has now been vindicated. The sequence of appearances that we now know is consistent with evolution, and the additional support for the common descent of animal phyla from new lines of inquiry that even Darwin could not have imagined only reinforces the brilliant predictive power of his insight. Today scientists might quibble about whether he assumed constant of rates of evolution with regard to the origin of animals. But despite this disagreement over rates of change, these fossils show that Darwin was right to suggest that life had been around for far longer before the beginning of the Cambrian than it has been since. As a result, it is appropriate to think of the Cambrian as a period of great phylogenetic diversification — what scientists call an evolutionary "radiation."

The Precambrian/Cambrian radiation and creationism: the original spin

Animals are incredibly complex and wonderful, and understanding their early evolution requires a full consideration of many different lines of evidence. Much is known, new data are appearing at an unprecedented rate, and yet many questions still excite our scientific curiosity. Scientists such as Stephen Jay Gould and Simon Conway Morris are valiantly striving to make these exotic animals and abstruse issues accessible to the public. It is a privilege to be able to witness all this excitement.

It is perhaps inevitable that those motivated by a nonscientific agenda will seek to extract snippets and sound bites from the scientific arguments, package them out of context, and feed them to the general public. This is what Fred Heeren did. Heeren is an anti-evolutionist writer who attended the Chengjiang meeting and then peddled his distorted version of the Cambrian radiation to the popular media, with obvious success (see Heeren 2000, an article in the Boston Globe).

Even if creationist misrepresentation of science is inevitable, it is nevertheless regrettable. Deep time was discovered 200 years ago and is now old news. Almost immediately, scientists recognized the sequential appearance of organisms in the geologic record, demonstrating the development of life's diversity through time, and this has never been seriously questioned in scientific circles. The world will be a better place when its human residents, in the brief flashes of time that each of us is privileged to experience, celebrate what science tells us about our place in nature.

Nigel Hughes is Associate Professor of Geology at the University of California, Riverside. He is a paleontologist interested in the evolution of trilobites and other early animals, and in the geological history of the Himalayas.

Dinosaurs and Birds — an Update

Reports of the National Center for Science Education

Title:

Dinosaurs and Birds — an Update

Author(s):

Kevin Padian, NCSE President

Volume:

20

Issue:

5

Year:

2000

Date:

September–October

Page(s):

28–31

This version might differ slightly from the print publication.

In a short paper in Nature, John Ostrom (1973) first laid out a case for the descent of birds from theropod dinosaurs. At the time, other ideas had recently been proposed, linking birds to crocodiles or to a more vaguely defined group of archosaurs (the group that includes birds, dinosaurs, crocodiles, pterosaurs, and many extinct relatives). Although all three hypotheses had early proponents, only the dinosaur-bird hypothesis survived the decade, mainly because (1) the evidence was convincing, (2) the hypothesis survived repeated tests using cladistic analysis, and (3) the alternatives were too vaguely phrased, there was no convincing evidence for them, and they failed repeated cladistic testing. The public tends to think that there is a substantial controversy among scientists about the ancestry of birds, partly because the public does not understand cladistics and partly because cladistics is rejected as a method by the opponents of the dinosaur-bird hypothesis.

What, then, is cladistics? Cladistics, or phylogenetic systematics, is a way of analyzing relationships that was first brought to the fore in the late 1960s, although it had been proposed in Germany decades earlier. By the early 1980s, it had demonstrated its practical and theoretical value to enough of the community of systematists that its methods became commonplace in studies of all branches of organisms, in most top scientific journals, and in the National Science Foundation's decisions about awards in systematic biology. Its influence has grown in succeeding years to the point that statements about evolutionary relationships are no longer taken seriously in the community of systematists unless backed by a cladistic analysis. This is true regardless of the type of organism and regardless of whether the postulated relationship is based on morphology, molecules, behavior, or fossils.

There is no guarantee that any given cladogram will not be revised or overturned by further study (new techniques are constantly being developed and revised); they are hypotheses that are meant to be tested, after all. But cladograms, unlike any other kind of evolutionary hypothesis of relationships, are explicit in their methods and the data on which they are based, and are testable. This gives them practical value. And because they restrict evidence to new, unique evolutionary features as a way of determining relationships among closest relatives, they are more consistent theoretically with the expectations of evolution than any other method.

Cladistics and its Critics

Critics of cladistics (those who still remain), or critics of the dinosaur-bird hypothesis, claim that cladistics has become dogma. To understand how people in the field respond to this complaint, I suggest the following analogy.

Most people accept, on practical as well as theoretical grounds, that medical imaging (X-rays, CAT scans, and so on) shows what is going on inside a patient. Before X-rays, physicians had to operate (dangerous and painful) or feel around on the outside and infer the patient's condition. With modern medical imaging, physicians can easily see an intestinal blockage, a tumor, or a fracture. The people who object to cladistics, decades after its general adoption, are like doctors who would rather feel around to diagnose the problem. It is not that they are necessarily wrong, but now we have better ways to diagnose ailments. If you were sick, would you rather have your doctor just feel around, or use an imaging technique such as an X-ray? At least as a second opinion?

Another criticism, focused on the widespread use of computers in cladistic analysis, is that cladistics is just "garbage in, garbage out". Not so: the computer does not do the thinking for the scientist. The scientist determines which characters and organisms to choose and which states are primitive and derived. The computer's role is just to do electronically what takes much longer to do by hand: namely, sorting out the shortest and simplest evolutionary "family" trees for further testing. It is exactly analogous to using a calculator instead of pencil and paper to add a long list of figures. And every cladistic analysis contains a list of the characters and organisms used and how the character states were coded, so anyone can run the analysis again - with variations, if desired.

Why the emphasis on methods in an article that is supposed to be an update on the dinosaur-bird hypothesis? Because every couple of months - or so it seems - there is some kind of challenge to the hypothesis, mounted by the same cast of characters. Well, fine; science is built on challenges to what we think we know. But when do we start to decide that a hypothesis is pretty robust to all this testing, and what standards of testing should we require? Although it has been over 25 years since Ostrom put forth the dinosaur-bird hypothesis, its opponents have yet to propose an alternative, testable hypothesis. So far not a single one of these opponents has ever - and I doubt they ever will - come out and said, "here's another animal or group of animals that we propose as closer to birds than the theropod dinosaurs, and here are the reasons." Their hypothesis is simply that the dinosaur-bird hypothesis is wrong. All the proposed similarities of birds and dinosaurs are mistakes and delusions.

Opponents also claim that the dinosaur-bird hypothesis is dogma, apparently on the grounds that those who accept it have not accepted the opponents' arguments for rejecting it. But science does not require unanimity, it does not force agreement, and it does not settle issues by vote. Some geologists went to their graves not accepting that the continents move. Science progresses nonetheless, by the accumulation of evidence and the testing of hypotheses that account for it. Today it is difficult to find an article in geology that begins by allowing that plate tectonics is only one possible model among many other equally plausible ones - even though 40 years ago the theory was hotly contested.

Well, what methods and tests are the anti-theropod critics using? Not cladistics: they do not use cladistics, because every time someone does a cladistic analysis, birds come out most closely related to theropod dinosaurs. The critics often admit their aversion to cladistics, but even when they do not, their papers speak for them: not a single real cladogram has appeared in any of their works.

Okay, we can all agree that any hegemony of method can be challenged. But in science, we do need methods. What, then, do they propose in place of cladistics? The answer is a resounding silence. They will not say what methods they are using, and so it is impossible for anyone to test their statements. Occasionally, they claim that they do not need methods because they have the crucial evidence to falsify the dinosaur-bird hypothesis. Luis Chiappe and I dealt with these objections in several publications, including our 1998 article in Scientific American (Padian and Chiappe 1998a) and a longer, more technical one in Biological Reviews in the same year (Padian and Chiappe 1998b).

There are two larger points of interest here. I am often asked, by other scientists, by reporters, and by members of the public who are just interested in questions about dinosaurs and evolution, "So what is it with these anti-theropod people? It sounds like you are arguing with creationists." And here, especially for the NCSE audience, I would like to demur on this comparison. It is intellectually dissonant to mention these two groups in the same sentence, because obviously the dissenters to the dinosaur-bird hypothesis are competent scientists who accept evolution. But the comparison appears to recur because if you have no alternative hypothesis to test scientifically, and you do not accept the methods of the field yet have no alternative methods that can be used, at some point observers will begin to wonder about the scientific basis for your statements. I think, in fairness to these dissenters, that they hold that evolutionary processes, as they understand them, would not be able to produce birds from dinosaurs; so the evolutionary patterns that we see in cladograms must be wrong.

The second point has to do with public education. Why does the public not understand the methodological basis of this dispute? The answer is two-pronged. First, the reporters assigned to cover the story do not make the issue clear to the public. This is because most of them do not understand it; they think that the dispute is largely motivated by personalities and politics in the absence of definitive evidence. It could be improved if reporters would explain at least a bit about the methods and standards of evidence, as opposed to "he said, she said" journalism. But after all, they are journalists, and we cannot expect them to be scientists too, any more than scientists can be competent reporters.

The second reason, which encompasses the first, is that even though cladistic analysis has been the standard for the field for two decades, it is almost unknown to the general public. Textbooks continue to teach the outworn Linnaean system and to portray taxonomy as some kind of art, instead of as a process that arranges organisms according to scientifically tested hypotheses about the evolutionary changes that produced a variety of descendants from a common ancestor.

Recent Developments in Bird Evolution

Recent developments illustrate some problems faced by the opponents of the dinosaur-bird hypothesis. Most readers will have heard that several kinds of feathered dinosaurs have been recently discovered in the Early Cretaceous deposits of Liaoning Province, China. They belong to several distinct groups within the broader group of coelurosaurian dinosaurs - the group to which all systematic analyses conclude that birds belong.

What sort of feathers do these dinosaurs have? Well, they have two kinds of integumentary structures. One kind produces a thick, relatively short and dense pelage made up of fibrous, filamentous structures that appear all over the body. These, claim some opponents of the dinosaur-bird hypothesis, are merely collagen (a common connective tissue in skin). However, molecular analysis shows that these structures are made of keratin - and not just any keratin, but the beta-keratin that feathers have (and, equally important, not the alpha-keratin that make up the scales of today's reptiles). Dinosaurs that have this kind of integumentary structure include several coelurosaurian theropods, such as the compsognathid Sinosauropteryx, the therizinosaurid Beipiaosaurus, and the dromaeosaur Saurornithosaurus (listed in order of their closeness to birds). More feathered coelurosaurs continue to be discovered and described in the scientific literature.

The second kind of integument is true feathers, which have a central shaft, two vanes, and barbs. These true feathers are attached to the forelimbs and tail just as the feathers of Archaeopteryx and living birds are. True feathers are found in the oviraptorosaur Caudipteryx and another form, Protarchaeopteryx, which are coelurosaurs. Opponents of the dinosaur-bird hypothesis have claimed that these are merely birds that have given up the ability to fly, but because they have not performed a phylogenetic analysis of any sort, they have no support for this assertion.

Opponents of the dinosaur-bird hypothesis keep publishing objections that are based on alternative interpretations of single features or specimens, which by themselves do not falsify the dinosaur-bird hypothesis. The most recent is a re-interpretation of Longisquama, an enigmatic reptile of undetermined relationship that occurs in the Triassic of Kazakhstan. It has been known for 30 years, but what is most interesting about Longisquama are the long oblong structures that appear to emanate from its vertebral column. Each of these structures has a central stalk that separates two flat, semi-elliptical surfaces. There are no barbs, but there are some features emanating from the stalk, wavy in contour, directed proximally near the base of the stalk and distally near its end. The entire structure is surrounded by a perimeter reminiscent of a rubber band.

The re-interpretation of this specimen as possessing true feathers (Jones and others 2000) was supposed to overturn the dinosaur-bird hypothesis, according to an aggressive press release and statements made for the benefit of the media, but for the most part it just left paleontologists scratching their heads. No one appeared to be postulating Longisquama as the closest relative of birds, so what were we supposed to learn from this publication? Perhaps we were to be admonished that the fossil record is rich enough to contain plenty of surprises, and so we should not be so confident in the dinosaur-bird hypothesis. Okay, caution taken. Now, what is the alternative hypothesis? And what is the method used to frame it?

Well, there is no hypothesis, and there is no method. Two major problems in the re-interpretation of Longisquama indicate the pitfalls of the "alternative" approach. First, the opponents of the dinosaur-bird hypothesis who published this paper asserted that Longisquama was an archosaur, but it is not. Archosaurs (by definition) include birds and crocodiles and all descendants of their closest common ancestor. No analysis yet has placed Longisquama anywhere near this group. Rather, it is apparently somewhere within Sauria, the broad group that includes living lizards and snakes, Sphenodon, crocodiles, birds, and all the descendants of their most recent common ancestor. The specimens preserve too few features to be much more specific. So it is difficult for them to propose that this animal had anything to do with the origin of birds.

The second error is the assertion that Longisquama had true feathers (Reisz and Sues 2000). Reporters found it difficult to get anyone else to agree with this (Stokstad 2000). The two most noted experts on feather structure and development rejected the idea, and one opined that the paper would not have been published in even a third-rate ornithological journal. As noted above, the similarities to feathers are superficial at best. Why, then, did the paper receive such attention in the popular and scientific press? Well, scientific journalism, especially in high-profile journals, is not above a bit of the "Man Bites Dog" mentality; there is competition to report on what seems new and exciting, even in the news sections of peer-reviewed publications.

Let me propose a litmus test. Next time you encounter a newspaper or television story on this or any scientific issue, get to the bottom of it with two questions: (1) What exact hypothesis is being proposed here to supplant another one (and it cannot be simply that the first hypothesis is wrong: we assume that in all tests)? (2) What methods are being used, if not the standard methods in the field, and how do we know that these are better than the standard methods? If and when the opponents of the dinosaur-bird hypothesis manage to give satisfactory answers to these two questions, they will be taken seriously.

How do you convince a creationist that a fossil is a transitional fossil? Give up? It is a trick question. You cannot do it. There is no convincing someone who has his mind made up already. But sometimes, it is even worse. Sometimes, when you point out a fossil that falls into the middle of a gap and is a superb morphological and chronological intermediate, you are met with the response: "Well, now you have two gaps where you only had one before! You are losing ground!"

One of the favorite anti-evolutionist challenges to the existence of transitional fossils is the supposed lack of transitional forms in the evolution of the whales. Duane Gish of the Institute for Creation Research (ICR) regularly trots out the "bossie-to-blowhole" transition to ridicule the idea that whales could have evolved from terrestrial, hooved ancestors.

There simply are no transitional forms in the fossil record between the marine mammals and their supposed land mammal ancestors . . . It is quite entertaining, starting with cows, pigs, or buffaloes, to attempt to visualize what the intermediates may have looked life. Starting with a cow, one could even imagine one line of descent which prematurely became extinct, due to what might be called an “udder failure” (Gish 1985: 78-9).

Of course, for many years the fossil record for the whales was quite spotty, but now there are numerous transitional forms that illustrate the pathway of whale evolution.

Recent discoveries of fossil whales provide the evidence that will convince an honest skeptic. However, evolutionary biology predicts more than just the existence of fossil ancestors with certain characteristics — it also predicts that all other biological disciplines should also revels patterns of similarity among whales, their ancestors, and other mammals correlated with evolutionary relatedness between groups. It should be no surprise that this is what we find, and since the findings in one biological discipline, say biochemistry, is derived without reference to the findings in another, say comparative anatomy, scientists consider these different fields to provide independent evidence of the evolution of whales. As expected, these independent lines of evidence all confirm the pattern of whale evolution that we would anticipate in the fossil record.

To illustrate this approach, I will present the evidence from multiple fields for the origin of the whales from terrestrial mammals. This paper will examine mutually reinforcing evidence from nine independent areas of research. Of course, as a starting point, we need to describe what makes a whale a whale.

What is a whale?

A whale is first and foremost, a mammal — a warm-blooded vertebrate that uses its high metabolism to generate heat and regulate its internal temperature. Female whales bear live young, which they nurse from mammary glands. Although adult whales have no covering of body hair, they acquire body hair temporarily as fetuses, and some adult whales have sensory bristles around their mouths. These features are unequivocally mammalian.

But a whale is a very specialized mammal with many unique characters that are not shared with other mammals — many of these are not even shared with other marine mammals such as sirenians (manatees and dugongs) and pinnipeds (seals, sea lions, and walruses). For example, whales have streamlined bodies that are thick and rounded, unlike the generally slim, elongated bodies of fishes. A whale's tail has horizontal flukes, which are its sole means of propulsion through the water. The dorsal fin is stiffened by connective tissue, but is fleshy and entirely without supporting bones.

The neck vertebrae of the whale are shortened and at least partly fused into a single bony mass. The vertebrae behind the neck are numerous and very similar to one another; the bony processes that connect the vertebrae are greatly reduced, allowing the back to be very flexible and to produce powerful thrusts from the tail flukes. The flippers that allow the whale to steer are composed of flattened and shortened arm bones, flat, disk-like wrist bones, and multiple elongated fingers. The elbow joint is virtually immobile, making the flipper rigid. In the shoulder girdle, the shoulder blade is flattened, and there is no clavicle. A few species of whales still possess a vestigial pelvis, and some have greatly reduced and nonfunctional hindlimbs.

The rib cage is very mobile — in some species, the ribs are entirely separated from the vertebral column — which allows the chest to expand greatly when the whale is breathing in and allows the thorax to compress at depth when the whale is diving deeply.

The skull also has a set of features unique among mammals. The jaws extend forward, giving whales their characteristically long head, and the two front-most bones of the upper jaw (the maxillary and premaxillary) are "telescoped" rearward, sometimes entirely covering the top of the skull. The rearward migration of these bones is the process by which the nasal openings have moved to the top of the skull, creating blowholes and shifting the brain and the auditory apparatus to the back of the skull. The odontocetes (toothed whales) have a single blowhole, while the mysticetes (baleen whales) have paired blowholes.

In the odontocetes, there is a pronounced asymmetry in the telescoped bones and the blowhole that provides a natural means of classification. Although teeth often occur in fetal mysticetes, only odontocetes exhibit teeth as adults. These teeth are always simple cones or pegs; they are not differentiated by region or function as teeth are in other mammals. (Whales cannot chew their food; it is ground up instead in a forestomach, or muscular crop, containing stones.)

Unlike the rest of the mammals, whales have no tear glands, no skin glands, and no olfactory sense. Their hearing is acute but the ear has no external opening. Hearing occurs via vibrations transmitted to a heavy, shell-like bone formed by fusion of skull bones (the periotic and auditory bullae).

These, then, are the major features of whales. Some clearly show the distinctive adaptations imposed on whales by their commitment to marine living; others clearly link the whales to their terrestrial ancestors. Others show the traces of descent from a terrestrial ancestor in common with several ancient and modern species. From all these features together, we can reconstruct the pathway that whale evolution took from a terrestrial ancestor to a modern whale confined to deep oceans.

Thinking about the ancestry of the whale

In 1693, John Ray recorded his realization that whales are mammals based on the similarity of whales to terrestrial mammals (Barnes 1984). The pre-Darwinian scientific discussion revolved around whether whales were descended from or ancestral to terrestrial mammals. Darwin (1859) suggested that whales arose from bears, sketching a scenario in which selective pressures might cause bears to evolve into whales; embarrassed by criticism, he removed his hypothetical swimming bears from later editions of the Origin (Gould 1995).

Later, Flower (1883) recognized that the whales have persistent rudimentary and vestigial features characteristic of terrestrial mammals, thus confirming that the direction of descent was from terrestrial to marine species. On the basis of morphology, Flower also linked whales with the ungulates; he seems to have been the first person to do so.

Early in the 20th century, Eberhard Fraas and Charles Andrews suggested that creodonts (primitive carnivores, now extinct) were the ancestors of whales (Barnes 1984). Later, WD Matthew of the American Museum of Natural History postulated that whales descended from insectivores, but his idea never gained much support (Barnes 1984). Later still, Everhard Johannes Slijper tried to combine the two ideas, claiming that whales descended from what Barnes aptly called "creodonts-cum-insectivores". However, no such animal has ever been found. More recently, Van Valen (1966) and Szalay (1969) associated early whales with mesonychid condylarths (a now-extinct group of primitive carnivorous ungulates, none bigger than a wolf) on the basis of dental characters. More recent evidence confirms their assessment. Thus Flower was basically right.

The evidence

The evidence that whales descended from terrestrial mammals is here divided into nine independent parts: paleontological, morphological, molecular biological, vestigial, embryological, geochemical, paleoenvironmental, paleobiogeographical, and chronological. Although my summary of the evidence is not exhaustive, it shows that the current view of whale evolution is supported by scientific research in several distinct disciplines.

1. Paleontological evidence

The paleontological evidence comes from studying the fossil sequence from terrestrial mammals through more and more whale-like forms until the appearance of modern whales. Although the early whales (Archaeocetes) exhibit greater diversity than I have space to discuss here, the examples in this section represent the trends that we see in this taxon. Although there are two modern suborders of whales (Odontocetes and Mysticetes), this discussion will focus on the origin of the whales as an order of mammals, and set aside the issues related to the diversification into suborders.

Sinonyx

We start with Sinonyx, a wolf-sized mesonychid (a primitive ungulate from the order Condylarthra, which gave rise to artiodactyls, perissodactyls, proboscideans, and so on) from the late Paleocene, about 60 million years ago. The characters that link Sinonyx to the whales, thus indicating that they are relatives, include an elongated muzzle, an enlarged jugular foramen, and a short basicranium (Zhou and others 1995). The tooth count was the primitive mammalian number (44); the teeth were differentiated as are the heterodont teeth of today's mammals. The molars were very narrow shearing teeth, especially in the lower jaw, but possessed multiple cusps. The elongation of the muzzle is often associated with hunting fish — all fish-hunting whales, as well as dolphins, have elongated muzzles. These features were atypical of mesonychids, indicating that Sinonyx was already developing the adaptations that later became the basis of the whales' specialized way of life.

Zhou and others (1995) published this reconstruction of the skull of Sinonyx jiashanensis (redrawn for RNCSE by Janet Dreyer).

Pakicetus

The next fossil in the sequence, Pakicetus, is the oldest cetacean, and the first known archaeocete. It is from the early Eocene of Pakistan, about 52 million years ago (Gingerich and others 1983). Although it is known only from fragmentary skull remains, those remains are very diagnostic, and they are definitely intermediate between Sinonyxand later whales. This is especially the case for the teeth. The upper and lower molars, which have multiple cusps, are still similar to those of Sinonyx, but the premolars have become simple triangular teeth composed of a single cusp serrated on its front and back edges. The teeth of later whales show even more simplification into simple serrated triangles, like those of carnivorous sharks, indicating that Pakicetus's teeth were adapted to hunting fish.

Gingrich and others (1983) published this reconstruction of the skull of Pakicetus inachus (redrawn for RNCSE by Janet Dreyer).

A well-preserved cranium shows that Pakicetus was definitely a cetacean with a narrow braincase, a high, narrow sagittal crest, and prominent lambdoidal crests. Gingerich and others (1983) reconstructed a composite skull that was about 35 centimeters long. Pakicetus did not hear well underwater. Its skull had neither dense tympanic bullae nor sinuses isolating the left auditory area from the right one — an adaptation of later whales that allows directional hearing under water and prevents transmission of sounds through the skull (Gingerich and others 1983). All living whales have foam-filled sinuses along with dense tympanic bullae that create an impedance contrast so they can separate sounds arriving from different directions. There is also no evidence in Pakicetus of vascularization of the middle ear, which is necessary to regulate the pressure within the middle ear during diving (Gingerich and others 1983). Therefore, Pakicetus was probably incapable of achieving dives of any significant depth. This paleontological assessment of the ecological niche of Pakicetus is entirely consistent with the geochemical and paleoenvironmental evidence. When it came to hearing, Pakicetus was more terrestrial than aquatic, but the shape of its skull was definitely cetacean, and its teeth were between the ancestral and modern states.

Ambulocetus

In the same area that Pakicetus was found, but in sediments about 120 meters higher, Thewissen and colleagues (1994) discovered Ambulocetus natans, "the walking whale that swims", in 1992. Dating from the early to middle Eocene, about 50 million years ago, Ambulocetus is a truly amazing fossil. It was clearly a cetacean, but it also had functional legs and a skeleton that still allowed some degree of terrestrial walking. The conclusion that Ambulocetus could walk by using the hind limbs is supported by its having a large, stout femur. However, because the femur did not have the requisite large attachment points for walking muscles, it could not have been a very efficient walker. Probably it could walk only in the way that modern sea lions can walk — by rotating the hind feet forward and waddling along the ground with the assistance of their forefeet and spinal flexion. When walking, its huge front feet must have pointed laterally to a fair degree since, if they had pointed forward, they would have interfered with each other.

The forelimbs were also intermediate in both structure and function. The ulna and the radius were strong and capable of carrying the weight of the animal on land. The strong elbow was strong but it was inclined rearward, making possible rearward thrusts of the forearm for swimming. However, the wrists, unlike those of modern whales, were flexible.

It is obvious from the anatomy of the spinal column that Ambulocetus must have swum with its spine swaying up and down, propelled by its back feet, oriented to the rear. As with other aquatic mammals using this method of swimming, the back feet were quite large. Unusually, the toes of the back feet terminated in hooves, thus advertising the ungulate ancestry of the animal. The only tail vertebra found is long, making it likely that the tail was also long. The cervical vertebrae were relatively long, compared to those of modern whales; Ambulocetus must have had a flexible neck.

Ambulocetus's skull was quite cetacean (Novacek 1994). It had a long muzzle, teeth that were very similar to later archaeocetes, a reduced zygomatic arch, and a tympanic bulla (which supports the eardrum) that was poorly attached to the skull. Although Ambulocetus apparently lacked a blowhole, the other skull features qualify Ambulocetus as a cetacean. The post-cranial features are clearly in transitional adaptation to the aquatic environment. Thus Ambulocetus is best described as an amphibious, sea-lion-sized fish-eater that was not yet totally disconnected from the terrestrial life of its ancestors.

Rodhocetus

In the middle Eocene (46-7 million years ago) Rodhocetus took all of these changes even further, yet still retained a number of primitive terrestrial features (Gingerich and others 1994). It is the earliest archaeocete of which all of the thoracic, lumbar, and sacral vertebrae have been preserved. The lumbar vertebrae had higher neural spines than in earlier whales. The size of these extensions on the top of the vertebrae where muscles are attached indicate that Rodhocetus had developed a powerful tail for swimming.

Gingrich and others (1994) published this reconstruction of the skeleton of Rodhocetus kasrani (redrawn for RNCSE by Janet Dreyer).

Elsewhere along the spine, the four large sacral vertebrae were unfused. This gave the spine more flexibility and allowed a more powerful thrust while swimming. It is also likely that Rodhocetus had a tail fluke, although such a feature is not preserved in the known fossils: it possessed features — shortened cervical vertebrae, heavy and robust proximal tail vertebrae, and large dorsal spines on the lumbar vertebrae for large tail and other axial muscle attachments — that are associated in modern whales with the development and use of tail flukes. All in all, Rodhocetus must have been a very good tail-swimmer, and it is the earliest fossil whale committed to this manner of swimming.

The pelvis of Rodhocetus was smaller than that of its predecessors, but it was still connected to the sacral vertebrae, meaning that Rodhocetus could still walk on land to some degree. However, the ilium of the pelvis was short compared to that of the mesonychids, making for a less powerful muscular thrust from the hip during walking, and the femur was about 1/3 shorter than Ambulocetus’s, so Rodhocetus probably could not get around as well on land as its predecessors (Gingerich and others 1994).

Rodhocetus's skull was rather large compared to the rest of the skeleton. The premaxillae and dentaries had extended forward even more than its predecessors’, elongating the skull and making it even more cetacean. The molars have higher crowns than in earlier whales and are greatly simplified. The lower molars are higher than they are wide. There is a reduced differentiation among the teeth. For the first time, the nostrils have moved back along the snout and are located above the canine teeth, showing blowhole evolution. The auditory bullae are large and made of dense bone (characteristics unique to cetaceans), but they apparently did not contain the sinuses typical of later whales, making it questionable whether Rodhocetus possessed directional hearing underwater.

Overall, Rodhocetus showed improvements over earlier whales by virtue of its deep, slim thorax, longer head, greater vertebral flexibility, and expanded tail-related musculature. The increase in flexibility and strength in the back and tail with the accompanying decrease in the strength and size of the limbs indicated that it was a good tail-swimmer with a reduced ability to walk on land.

Basilosaurus

The particularly well-known fossil whale Basilosaurus represents the next evolutionary grade in whale evolution (Gingerich 1994). It lived during the late Eocene and latest part of the middle Eocene (35-45 million years ago). Basilosaurus was a long, thin, serpentine animal that was originally thought to have been the remains of a sea serpent (hence it is name, which actually means "king lizard"). Its extreme body length (about 15 meters) appears to be due to a feature unique among whales; its 67 vertebrae are so long compared to other whales of the time and to modern whales that it probably represents a specialization that sets it apart from the lineage that gave rise to modern whales.

What makes Basilosaurus a particularly interesting whale, however, is the distinctive anatomy of its hind limbs (Gingerich and others 1990). It had a nearly complete pelvic girdle and set of hindlimb bones. The limbs were too small for effective propulsion, less than 60 cm long on this 15-meter-long animal, and the pelvic girdle was completely isolated from the spine so that weight-bearing was impossible. Reconstructions of the animal have placed its legs external to the body — a configuration that would represent an important intermediate form in whale evolution.

Although no tail fluke has ever been found (since tail flukes contain no bones and are unlikely to fossilize), Gingerich and others (1990) noted that Basilosaurus's vertebral column shares characteristics of whales that do have tail flukes. The tail and cervical vertebrae are shorter than those of the thoracic and lumbar regions, and Gingerich and others (1990) take these vertebral proportions as evidence that Basilosaurus probably also had a tail fluke.

Further evidence that Basilosaurus spent most of its time in the water comes from another important change in the skull. This animal had a large single nostril that had migrated a short distance back to a point corresponding to the back third of the dental array. The movement from the forward extreme of the snout to the a position nearer the top of the head is characteristic of only those mammals that live in marine or aquatic environments.

Dorudon

Dorudon was a contemporary of Basilosaurus in the late Eocene (about 40 million years ago) and probably represents the group most likely to be ancestral to modern whales (Gingerich 1994). Dorudon lacked the elongated vertebrae of Basilosaurus and was much smaller (about 4-5 meters in length). Dorudon’s dentition was similar to Basilosaurus’s; its cranium, compared to the skulls of Basilosaurus and the previous whales, was somewhat vaulted (Kellogg 1936). Dorudon also did not yet have the skull anatomy that indicates the presence of the apparatus necessary for echolocation (Barnes 1984).

Gingrich and Uhen (1996) published this reconstruction of the skeleton of Dorudon atrox (redrawn for RNCSE by Janet Dreyer).

Basilosaurus and Dorudon were fully aquatic whales (like Basilosaurus, Dorudon had very small hind limbs that may have projected slightly beyond the body wall). They were no longer tied to the land; in fact, they would not have been able to move around on land at all. Their size and their lack of limbs that could support their weight made them obligate aquatic mammals, a trend that is elaborated and reinforced by subsequent whale taxa.

Clearly, even if we look only at the paleontological evidence, the creationist claim of "No fossil intermediates!" is wrong. In fact, in the case of whales, we have several, beautifully arranged in morphological and chronological order.

In summarizing the paleontological evidence, we have noted the consistent changes that indicate a series of adaptations from more terrestrial to more aquatic environments as we move from the most ancestral to the most recent species. These changes affect the shape of the skull, the shape of the teeth, the position of the nostrils, the size and structure of both the forelimbs and the hindlimbs, the size and shape of the tail, and the structure of the middle ear as it relates to directional hearing underwater and diving. The paleontological evidence records a history of increasing adaptation to life in the water — not just to any way of life in the water, but to life as lived by contemporary whales.

2. Morphological evidence

The examination of the morphological characteristics shared by the fossil whales and living ungulates makes their common ancestry even clearer. For example, the anatomy of the foot of Basilosaurus allies whales with artiodactyls (Gingerich and others 1990). The axis of foot symmetry in these fossil whales falls between the 3rd and 4th digits. This arrangement is called paraxonic and is characteristic of the artiodactyls, whales, and condylarths, and is rarely found in other groups (Wyss 1990).

Another example involves the incus (the "anvil" of the middle ear). The incus of Pakicetus, preserved in at least one specimen, is morphologically intermediate in all characters between the incus of modern whales and that of modern artiodactyls (Thewissen and Hussain 1993). Additionally, the joint between the malleus (hammer) and incus of most mammals is oriented at an angle between the middle and the front of the animal (rostromedially), while in modern whales and in ungulates, it is oriented at an angle between the side and the front (rostrolaterally). In Pakicetus, the first fossil cetacean, the joint is oriented rostrally (intermediate in position between the ancestral and derived conditions). Thus the joint has clearly rotated toward the middle from the ancestral condition in terrestrial mammals (Thewissen and Hussain 1993); Pakicetus provides us with a snapshot of the transition.

3. Molecular biological evidence

The hypothesis that whales are descended from terrestrial mammals predicts that living whales and closely related living terrestrial mammals should show similarities in their molecular biology roughly in proportion to the recency of their common ancestor. That is, whales should be more similar in their molecular biology to groups of animals with which they share a more recent common ancestor than to other animals that exhibit convergent similarities in morphology, ecology, or behavior. In contrast, creationism lacks any scientific basis for predicting what the patterns of similarity should be, for there is no scientific way to predict how the creator decided to distribute molecular similarities among species.

Molecular studies by Goodman and others (1985) show that whales are more closely related to the ungulates than they are to all other mammals — a result consistent with evolutionary expectations. These studies examined myoglobin, lens alpha-crystallin A, and cytochrome c in a study of 46 different species of mammals. Miyamoto and Goodman (1986) later expanded the number of protein sequences by including alpha- and beta- hemoglobins and ribonuclease; they also increased the number of mammals included in the study to 72. The results were the same: the whales clearly are included among the ungulates. Other molecular studies on a variety of genes, proteins, and enzymes by Irwin and others (1991), Irwin and Arnason (1994), Milinkovitch (1992), Graur and Higgins (1994), Gatesy and others (1996), and Shimamura and others (1997) also identified the whales as closely related to the artiodactyls, although there are differences in the details among the studies.

By placing whales close to, and even firmly within, the Artiodactyls, these molecular studies confirm the predictions made by evolutionary theory. This pattern of biochemical similarities must be present if the whales and the ungulates, especially the Artiodactyls, share a close common ancestor. The fact that these similarities are present is therefore strong evidence for the common ancestry of whales and ungulates.

4. Vestigial evidence

The vestigial features of whales tell us two things. They tell us that whales, like so many other organisms, have features that make no sense from a design perspective — they have no current function, they require energy to produce and maintain, and they may be deleterious to the organism. They also tell us that whales carry a piece of their evolutionary past with them, highlighting a history of a terrestrial ancestry.

Modern whales often retain rod-like vestiges of pelvic bones, femora, and tibiae, all embedded within the musculature of their body walls. These bones are more pronounced in earlier species and less pronounced in later species. As the example of Basilosaurus shows, whales of intermediate age have intermediate-sized vestigial pelves and rear limb bones.

Whales also retain a number of vestigial structures in their organs of sensation. Modern whales have only vestigial olfactory nerves. Furthermore, in modern whales the auditory meatus (the exterior opening of the ear canal) is closed. In many, it is merely the size of a thin piece of string, about 1 mm in diameter, and often pinched off about midway. All whales have a number of small muscles devoted to nonexistent external ears, which are apparently a vestige of a time when they were able to move their ears — a behavior typically used by land animals for directional hearing.

The diaphragm in whales is vestigial and has very little muscle. Whales use the outward movement of the ribs to fill their lungs with air. Finally, Gould (1983) reported several occurrences of captured sperm whales with visible, protruding hind limbs. Similarly, dolphins have been spotted with tiny pelvic fins, although they probably were not supported by limb bones as in those rare sperm whales. And some whales, such as belugas, possess rudimentary ear pinnae — a feature that can serve no purpose in an animal with no external ear and that can reduce the animal's swimming efficiency by increasing hydrodynamic drag while swimming.

Although this list is by no means exhaustive, it is nonetheless clear that the whales have a wealth of vestigial features left over from their terrestrial ancestors.

5. Embryological evidence

Like the vestigial features, the embryological features also tells us two things. First, the whale embryo develops a number of features that it later abandons before it attains its final form. How can creationism explain such seemingly nonsensical process, building structures only to abandon them or to destroy them later? Darwin (1859) asked the same question. Would it not make more sense to have embryos attain their adult forms quickly and directly? It seems unreasonable for a perfect designer or creator to send the embryo along such a tortuous pathway, but evolution requires that new features are built on the foundation of previous features that it would modify or discard later.

Second, the embryology of the whale, examined in detail, also provides evidence for its terrestrial ancestry. As embryos no less than as adult animals, whales are junkyards, as it were, of old, discarded features that are of no further use to them. Many whales, while still in the womb, begin to develop body hair. Yet no modern whales retain any body hair after birth, except for some snout hairs and hairs around their blowholes used as sensory bristles in a few species. The fact that whales possess the genes for producing body hair shows that their ancestors had body hair. In other words, their ancestors were ordinary mammals.

In many embryonic whales, external hind limb buds are visible for a time but thendisappear as the whale grows larger. Also visible in the embryo are rudimentary ear pinnae, which disappear before birth (except in those that carry them as rare atavisms). And, in some whales, the olfactory lobes of the brain exist only in the fetus. The whale embryo starts off with its nostrils in the usual place for mammals, at the tip of the snout. But during development, the nostrils migrate to their final place at the top of the head to form the blowhole (or blowholes).

We can also understand evolution within the whales via their embryology. We know that the baleen whales evolved from the toothed whales: some embryos of the baleen whales begin to develop teeth. As with body hair, the teeth disappear before birth. Since there is no use for teeth in the womb, only inheritance from a common ancestor makes any sense; there is no reason for the intelligent designer or special creator to provide embryonic whales with teeth. So we have yet another independent field in complete accord with the overall thesis — that whales possess features that connect them with terrestrial mammalian ancestors, in particular the hoofed mammals.

6. Geochemical evidence

The earliest whales lived in freshwater habitats, but the ancestors of modern whales moved into saltwater habitats and thus had to adapt to drinking salt water. Since fresh water and salt water have somewhat different isotopic ratios of oxygen, we can predict that the transition will be recorded in the whales' skeletal remains — the most enduring of which are the teeth. Sure enough, fossil teeth from the earliest whales have lower ratios of heavy oxygen to light oxygen, indicating that the animals drank fresh water (Thewissen and others 1996). Later fossil whale teeth have higher ratios of heavy oxygen to light oxygen, indicating that they drank salt water. This absolutely reinforces the inference drawn from all the other evidence discussed here: the ancestors of modern whales adapted from terrestrial habitats to saltwater habitats by way of freshwater habitats.

7. Paleoenvironmental evidence

Evolution makes other predictions about the history of taxa based on the "big-picture" view of the fossils in a larger, environmental, context. The sequence of whale fossils and their changes should also relate to changes observed in the fossil records of other organisms at the same time and in similar environments. The fossils of other organisms associated with the whale fossils indicate the environment that the whales lived in. Furthermore, this evidence should be consistent with the evidence from the other areas of study. We should expect to find evidence for a series of transitional environments, from fully terrestrial to fully marine, occupied by the series of whale species in the fossil record.

The morphology of Sinonyx indicates that it was fully terrestrial. It should be no surprise, therefore, that its fossils are found associated with the fossils of other terrestrial animals. Pakicetus probably spent a lot of time in the water in search of food. Although the mammalian fauna found with Pakicetus consists of rodents, bats, various artiodactyls, perissodactyls and probiscideans, and even a primate (Gingerich and others 1983), there are also aquatic animals such as snails, fish, turtles and crocodilians. Moreover, the sediment associated with Pakicetus shows evidence of streaming or flowing, usually associated with soils that are carried by water. The paleoenvironmental evidence thus clearly shows that Pakicetus lived in the low-lying wet terrestrial environment, making occasional excursions into fresh water. Interestingly, both deciduous and permanent teeth of the animal are found in these sediments with about the same frequency, supporting the idea that Pakicetus gave birth on the land.

The sediments in which Ambulocetus was found contain leaf impressions as well as fossils of the turret-snail Turritella and other marine mollusks. Clearly, the presence of such fossils must mean that the Ambulocetus fossil was found in what was once a shallow sea — although leaves can be washed into the sea and fossilize there, marine mollusks would not be found on the land.

Rodhocetus is found in green shales deposited in the deep-neritic zone (equivalent to the outer part of the continental shelf). Because green shales are associated with fairly low-oxygen bottom waters, Rodhocetus must have lived at a greater water depth than any previous cetacean. The fact that it is found in association with planktonic foraminiferans and other microfossils agrees with this determination of water depth. Basilosaurus and Dorudon have been found in a variety of sediment types (Kellogg 1936), indicating that they were wide-ranging and capable of living in deep as well as shallow water.

From the paleoenvironmental evidence, we can clearly see that, as whales evolved, they made their way into deeper water and became progressively liberated from the terrestrial and near-shore environments.

8. Paleobiogeographic evidence

The geographic evidence is also consistent with the expected distributional patterns for the whale’s first appearance and later geographic expansion. We would expect terrestrial species to have a more restricted geographic distribution than marine species, which have essentially the whole ocean as their geographic range. The range of Sinonyx is restricted to central Asia. Specimens of Pakicetus have only been found in Pakistan; Ambulocetus and Rodhocetus seem to be similarly restricted. In contrast, Basilosaurus and Dorudon, representing the whales more adapted to living in the open sea, are found in a much wider area. Their fossils have been found as far away from southern Asia as Georgia, Louisiana, and British Columbia.

During the Eocene, most of the areas in which fossils of the later whales have been found were fairly close to one another. In fact, most of them are along the outer margin of an ancient sea called the Tethys, the remnants of which today are the Mediterranean, the Caspian, the Black, and the Aral Seas. The biogeographic distribution of fossil whales matches the pattern predicted by evolution: whales are initially found in a rather small geographic area and did not become distributed throughout the world until after they evolved into fully aquatic animals that were no longer tied to the land.

9. Chronological evidence

The final strand of evidence in our mutually consistent picture of whale origins comes from a consideration of why the whales originated when they did. Evolution is a response to environmental challenges and opportunities. During the early Cenozoic, mammals were presented with a new set of opportunities for radiation and diversification due, in part, to the vacuum left by mass extinctions at the close of the Cretaceous Period. Because the reptiles no longer predominated, there were new ways in which mammals could make a living.

In the specific case of whales, the swimming reptiles of the world's oceans could no longer keep the mammals at bay. Before the late-Cretaceous extinctions, the Mesozoic marine reptiles such as the plesiosaurs, ichthyosaurs, mosasaurs, and marine crocodiles might well have feasted upon any mammal that strayed off shore in search of food. Once those predators were gone, the evolution quickly produced mammals, including whales, that were as at home in the seas as they once were on land. The transition took some 10-15 million years to produce fully aquatic, deep-diving whales with directional underwater hearing. Evolution predicts that whales could not have successfully appeared and radiated before the Eocene, and that mammals should have radiated into marine environments as they did into a wide variety of other environments vacated by the reptiles at the end of the Cretaceous.

Conclusion

Taken together, all of this evidence points to only one conclusion — that whales evolved from terrestrial mammals. We have seen that there are nine independent areas of study that provide evidence that whales share a common ancestor with hoofed mammals. The power of evidence from independent areas of study that support the same conclusion makes refutation by special creation scenarios, personal incredulity, the argument from ignorance, or "intelligent design" scenarious entirely unreasonable. The only plausible scientific conclusion is that whales did evolve from terrestrial mammals. So no matter how much anti-evolutionists rant about how impossible it is for land-dwelling, furry mammals to evolve into fully aquatic whales, the evidence itself shouts them down. This is the power of using mutually reinforcing, independent lines of evidence. I hope that it will become a major weapon to strike down groundless anti-evolutionist objections and to support evolutionary thinking in the general public. This is how real science works, and we must emphasize the process of scientific inference as we point out the conclusions that scientists draw from the evidence — that the concordant predictions from independent fields of scientific study confirm the same pattern of whale ancestry.

Acknowledgements

I would like to thank Dr Philip Gingerich for his assistance with and review of this article.